1. Field of the Invention
The present invention relates generally to gas turbine engines, and more specifically, to coatings used to contain molten material within the core of the turbine engine.
2. Description of the Prior Art
The Federal Aviation Administration (FAA) requires that in the event of turbine engine fires, molten materials be confined, as much as possible, within the core of the turbine engine. This requirement is a necessary precautionary measure to insure that other aircraft components located both within and outside the confines of the turbine engine environment, such as fuel cells, flight control linkages and surfaces (including hydraulic control systems) are not exposed to molten materials that are expelled from the turbine engine. To meet this requirement, it is generally necessary to include shields, drip pans, blankets and other confining structures surrounding potentially hazardous areas of the turbine engine. These structures are specifically designed to stop and/or to cool molten materials coming into contact with the confining structures.
Improvement in the weight-to-performance characteristics of a turbine engine involves, in part, decreasing engine weight without sacrificing the strength and structural integrity of engine components, and thus, of the engine as a whole. To this end, titanium alloys have been incorporated in modern turbine engine components. Titanium is generally considered lighter in weight and relatively strong as compared to nickel alloys, which have also been used extensively in turbine engines. However, because titanium and titanium alloys would likely ignite if exposed to the high temperatures or pressures present in hot sections of the gas turbine engine, such as the high pressure compressor section of the engine, the use of titanium is limited to the cooler regions of the turbine engine. Use of titanium in hotter portions of the turbine engine would increases the probability of titanium ignition and would thus increase the need for additional safety devices to insure containment of potential engine fires.
The turbine engine temperature profile is progressive, in that it increases from a relatively low temperature at the engine intake to a much hotter temperature at the burner/turbine section of the engine and then decreases slightly to a temperature that is still relative high, at the engine exhaust. A median threshold temperature exists within the temperature profile of a turbine engine, at which titanium and titanium alloys, if located close to a portion of the engine that is at the median threshold temperature, would likely ignite, thereby causing a catastrophic engine failure. The median threshold temperature point may migrate longitudinally within the engine, due to changes in engine power demands, changes in the efficiency of the engine (i.e., engines that have been operated for many hours since overhaul versus engines that were recently overhauled may have a relocated temperature threshold), and minor malfunctions that may occur, including fan blade, turbine blade, or bearing failures, or due to aerodynamic heating.
Since titanium is employed in compressor rotors, stators, and casings (inner and outer) that can be located relatively close to the median threshold temperature point, drip pans are customarily used in these sections to contain molten titanium and other materials in the event of fire or overheating. The pans surround areas of the engine that could overheat and thereby expel molten materials through bleed holes in the compressor section. Bleed holes are used for drawing off compressed air for various aircraft systems, including hot air for deicing, and hot or cool air for aircraft cabin comfort. The pans are generally made of a material having a melting temperature substantially above the anticipated temperature of the molten material that may contact the pans. However, although the melting temperature of the pans may be higher than the temperature of the molten materials that can contact the pans, if the pan mass is insufficient, the molten material may soften and eventually burn through the pan. Once through the pan, the molten material may bum through the compressor casing, escaping the confines of the turbine engine.
The additional weight and bulk of safety devices such as steel drip pans increase the weight and size of the engine. This increased weight in turn affects the performance and efficiency of the aircraft on which the engine is installed. Accordingly, use of metal drip pans is contrary to the goal of reducing total engine weight to achieve increased aircraft economy and performance. In addition, where modern turbine engines with improved performance and economy of operation are desired for use on smaller aircraft, the elimination of steel drip pans may make what would otherwise be dimensionally too large an engine, sufficiently small to be used on a smaller aircraft. This advantage may be especially important in retrofitting a newer engine to an older aircraft that has a limited size engine compartment.
Accordingly, there is a need for a method and apparatus to contain molten materials within the turbine engine environment without adding significant weight to the engine and without requiring the space associated with conventional safety devices.